[ticket/11915] Improve Plupload integration

Changes that this implements:

- Replaces the current Plupload jQuery widget with our own custom uploader.
- All files are now consolidated into the same table under the attachments tab and are handled by the new uploader. This means that files that have been previewed/submitted can also be deleted without a page reload.
- Errors are displayed using phpBB's alert handler.
- The inline attachment bbcodes in the message are now updated on the client-side after a file is uploaded/deleted.
- Files start uploading immediately after they're selected. The "Start upload" button is no longer used.
- The uploader now checks for trigger_error() being called and displays the appropriate message to the user.
- The user can now provide a file comment or place the file inline immediately after it's uploaded.
- A link to view the file is now provided to the user, just as is the case when previewing.
- The drag-and-drop zone is now the message textarea.

http://tracker.phpbb.com/browse/PHPBB3-11915
http://tracker.phpbb.com/browse/PHPBB3-11961